1. Field of the Invention
The present invention is concerned with a process and an apparatus for coating a substrate, consisting of a strip of paper or paper lined with metal foil, with a polyethylene thermoplastic, in which the thermoplastic material is extruded to a thin film which, while it is still warm, is brought into contact with the substrate and, with simultaneous cooling to a temperature below the softening temperature of the plastic material, is pressed together with the substrate.
In the production of, for example, coated material for packing purposes, there is often used a layer of thermoplastic material, for example polyethylene, applied to a substrate where the thermoplastic material is extruded to a thin film which, while it is still in the molten or semi-molten state, is united with the substrate which may, for example, consist of a strip of cardboard or a metal foil.
In order to achieve good adhesion between the extruded thermoplastic coating and the substrate, they are pressed together between cooled pressure rollers, whereby the thermoplastic layer is brought into good contact with the substrate and at the same time is stabilized by cooling. When carrying out this coating process it often happens that the plastic material adheres to the cooling roll and that the coating material is in this case subjected to a force that is mainly at right angles to the coating surface. This force can easily cause a delamination of the material that is not necessarily confined to the boundary zone of the applied plastic layer. The delamination can be localized in the substrate, for example if this consists of paper or other cellular material.
2. Description of the Prior Art
Attempts have been made to avoid the above mentioned difficulty by wetting the cooling roll with water as well as by applying a coating of polytetrafluoroethylene, sold under the trade mark "Teflon", to the outer surface of the roll. The best result is achieved with the "Teflon" coated roll but delamination can occur even with such a cooling roll; moreover it has been found that the "Teflon" material cannot withstand for sufficiently long the mechanical and thermal stresses which arise at the outer surface of the cooling roller.
U.S. Pat. No. 3,215,578 granted Nov. 2, 1965 to J. N. Craver, Jr. describes and claims an apparatus and method for forming optically clear thermoplastic coated regenerated cellulose film. In this patent a cellulose film is prefabricated to have a moisture-proof wax coating on one surface, while the thermoplastic layer is extruded and in hot condition is brought together with the other surface of the cellulose film by means of pressure rolls. The pressure roll which contacts the moisture-proof surface of the cellulose film is rubber coated while the roll which contacts the thermoplastic layer is a metal chill roll. A liquid is applied to the rubber roll which contacts the cellulose layer, not the plastic film. In the claimed method of this patent, water is used as the liquid.
The invention of U.S. Pat. No. 3,215,578 is concerned with avoiding damage affecting optical clarity of the film and resulting from impressions in the cellulose layer, caused by irregularities in the rubber roll. The present invention is concerned with a different problem, preventing adhesion of a polyethylene film to the roll which contacts said film. To solve the present problem, a specific water-alcohol mixture is applied to a roll which contacts the polyethylene film, in contrast to the use of any kind of liquid, particularly water, in U.S. Pat. No. 3,215,578, which applies the liquid so as to contact the cellulose layer, not the polyethylene layer.
In accordance with U.S. Pat. No. 3,498,865 granted on Mar. 3, 1970 to Leon J. Paquin et al., paper is coated with polyethylene with use of a cooled roll in contact with the hot plastic. The cooled roll is coated with polytetrafluoroethylene to prevent the plastic from sticking to the roll. As described above, there have been found to be problems of durability of the "Teflon" coating.
U.S. Pat. No. 3,589,975 granted on June 29, 1971 to W. P. Andrews et al. is concerned with laminating plastic and metallic sheets, and does not disclose a means of avoiding adhesion of the plastic to a roll during cooling of the plastic.